1. Field of the Invention
The invention relates to closed impellers for centrifugal pumps, for conveying homogeneous liquids, particularly for use in motor vehicle cooling systems.
2. The Prior Art
In the prior art, various construction forms of closed impellers have been previously described.
For example, in DE patent 843 812, a construction form of an impeller for centrifugal pumps for conveying non-homogeneous substances (substances having an elevated gas or solid content) is previously described, which is characterized by a special channel and impeller wheel configuration, by means of which an attempt is made to cancel out the usual de-mixing effect of the centripetal field of the centrifugal pumps. The arrangement presented there is particularly characterized by at most four main blades, between which mixing blades are disposed on the outer circumference.
This arrangement represents a compromise for conveying non-homogeneous substances. However, optimal conveying of liquids, for example with regard to the degree of effectiveness, the pressure build-up, the characteristic line stability, etc., can by no means be implemented by means of this arrangement and blade wheel design presented in DE patent 843 812. Furthermore, such an impeller is cost-intensive in production, and also cannot in any case be produced in one piece, as a closed impeller.
Another construction form of a blade wheel for conveying non-homogeneous substances is previously described in CH patent 269 595. In this connection, an additional increase in cross-section from the entry towards the exit of the impeller channel is proposed for conveying conveyed media that contain gas, in practically all the consistencies that occur, along with a negative (by an angle α) blade overlap, in order to intentionally force a partial vacuum zone to occur in the channel, by means of detachment, which zone then allows conveying conveyed media that contain gas, in practically all the consistencies that occur.
However, even this arrangement by no means allows optimal conveying of homogeneous liquids, particularly with regard to its operational parameters, such as degree of effectiveness, suction behavior, pressure build-up, conveying height, characteristic line stability, etc.
All the closed impellers previously described in the prior art for conveying homogeneous liquids, whether in the construction form of radial blade wheels having blades curved in two or three dimensions, or in the construction form of Francis blade wheels, or diagonal blade wheels having blades spatially curved exclusively in three dimensions, always have an exit width (b2) of the blade channel that is reduced as compared with the entry width (b1), in section through the axis of rotation and the flow channel, i.e. in the meridian section, in order to bring about an increase in the flow velocity of the conveyed medium in the impeller.
One of these construction forms, with simply curved blades and a rectangular flow channel cross-section, is presented in DE patent 195 747, for example.
In the case of another closed impeller construction form, presented in DE patent 897 801, for example, the channel width remains the same at first, in the region of the flow entry, and then narrows towards the flow exit.
As a result of this exit width (b2) of the blade channel, which is reduced as compared with the entry width (b1) in the meridian section, the production of closed impellers for centrifugal pumps is connected with clearly increased production effort and expenditure as compared with the relatively uncomplicated production of open impeller forms for the centrifugal pump systems.
The open impeller construction forms are generally produced in one piece, using the plastic injection-molding method, from plastic, for example from a thermoplastic or duroplastic material.
The closed construction forms of impellers can also be produced from plastic.
However, for this purpose, an open blade wheel having blades curved in one or two dimensions must first be produced, which is then connected with a separately produced cover disk, to form a closed impeller, in a subsequent work step.
On the other hand, however, closed impellers having blades curved in one or two dimensions can also be produced in one piece, for example, as plastic injection-molded parts, using divided slides. It is true that the work step of assembly of the cover disk is eliminated, but then, clearly higher tool costs also necessarily occur.
In general, closed impellers having spatially curved blades, as compared with closed impellers having blades curved in two dimensions (e.g. having a rectangular flow channel cross-section, as presented in DE patent 195 747), have a larger specific diameter, poorer suction behavior, and also an increased risk of cavitation, at the same conveying performance.
In addition, the degree of hydraulic effectiveness of closed impellers having simply curved blades that can be reached amounts to maximally 70%.
In comparison with this, a degree of hydraulic effectiveness of up to 87% can be achieved with closed (rapid-running) impellers having spatially curved blades.
However, until now, the major hindrance for use of closed, rapid-running impellers having spatially curved blades on a large technical scale, for example for use as impellers in coolant pumps in engine and automobile construction, has always been their complicated and therefore cost-intensive production.
The impellers for coolant pumps, which are necessarily relatively small and furthermore have a very complicated structure, with their spatially curved blades, had to be produced in very cost-intensive manner, because of their complicated removal from a mold, predominantly as investment casting parts, using the lost wax process, or using the sand mold casting process, and were therefore unsuitable for large-scale use.
In addition, there was the problem that in the case of these relatively small construction forms, the possibilities for cleaning and polishing the casting surfaces inside the impeller are very limited, so that furthermore, the surface quality that could be achieved was also very greatly restricted.
As a result of this relatively great surface roughness, which necessarily remained due to the production method, the maximal degree of effectiveness that could be achieved was also very greatly impaired.
Therefore, the applicant of the present invention proposed a new type of construction form for closed, rapid-running impellers having spatially curved blades, in DE 197 42 023 B4, which is characterized not only by precisely defined blade shapes, a high level of blade surface quality, good concentricity properties, a high level of reliability, minimized effort and expenditure for production and assembly, as well as a high degree of effectiveness.
This construction form, which was proposed in DE 197 42 023 B4 and has proven itself many times in practical use, is characterized by a segmentation of the impeller, whereby the division of the impeller, in each instance, always takes place in the region of the blades, and in this connection leads to easily unmoldable impeller segments, which can also be produced from plastic, for example, in cost-advantageous manner.
These individual impeller segments are joined together, after their production, in a separate work step, to form an impeller having spatially curved blades, and in this connection are clamped together with one another by means of a bottom disk and/or a clamping ring.
In this connection, the impeller material plastic, which is mostly used in series production, allows not only cost-advantageous production of the individual components and a high surface quality that can be achieved in relatively cost-advantageous manner, but also low friction resistance, high corrosion resistance with regard to the conveyed medium, and good resistance to cavitation phenomena.
Cavitation occurs in the operational state of the impeller in regions having a low pressure level, i.e. at a pressure below the vapor pressure of the conveyed medium, but the effects of cavitation, i.e. implosion of the cavitation bubbles and the wear that results from this implosion, cavity erosion, always occurs only in the regions having an elevated pressure, i.e. mostly after the impeller.
The components and modules situated there, “downstream” from the impeller in the flow direction, are the pump housing and/or the control housing lid, the cylinder crankcase, the cylinder head, and the like, as the result of integration of the pump into the engine.
However, in modern engines, these “downstream” components are usually made from cast aluminum.
In comparison with plastic, aluminum castings have much poorer resistance to cavitation erosion, so that cavitation that builds up in the impeller in regions having a low pressure level leads to damage caused by cavitation erosion of the components and modules made from cast aluminum that follow the impeller, and this can then lead to total failure of these components/modules after extended long-term operation.